In 1999, lodenosine [6-amino-9-(2,3-dideoxy-2-fluoro-beta-D-threo-pentofuranosyl)-9H-purine)] , an antiretroviral agent discovered and developed in this laboratory for the treatment ofAIDS was discontinued from phase I/II clinical trials by the FDA due to liver toxicity. The LMCH is currently interested in determining whether the liver toxicity reported was the result of a 0.1 0.3% contamination of the bulk drug with 6-amino-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl) 9H-purine (2-F-ara-A). By the nature of its structure, 2-F-ara-A is expected to be several orders of magnitude better substrate than lodenosine for cellular polymerases. The effect on cellular polymerases, particularly from mitochondria, could explain the clinical toxicity. The synthesis of the contaminant was performed to help with these studies and the synthesis of the 5-triphosphate metabolite is currently in progress. At the mechanistic level, a conformational study of the active site of HIV reverse transcriptase (RT) utilizing the recent X-ray structure of the ternary complex (RT-DNA-dNTP) helped define the role of Tyrosine 115 in modulating the antiretroviral activity of lodenosine and its 2-alpha-fluoro diastereoisomer (Biochemistry 2000, 29, 11205-11215). Following the lead that 4-C-alpha-ethynyl-2-deoxynucleosides are highly active against multi drug resistant HIV, the LMCH embarked in the synthesis of the corresponding 2,3-dideoxy version of these nucleosides. The cytosine analogue was selected as the first target. The synthesis of the racemic analogue is nearing completion, and both D- and L-isomers will be investigated. A novel series of conformationally constrained 3-oxobicyclo[3.1.0]hexane dideoxynucleoside analogues that comprised both alpha- and beta-anomers of the natural bases (A, C, T, G) was completed and tested. No antiretroviral activity was detected. However, the unusual cytotoxicity of the alpha-cytosine analogue will be investigated in the NCI 60 cell line screen for antitumor activity. AIDS title: Fluorodideoxynucleosides as reverse transcriptase inhibitors for the treatment of AIDS. 4-C-alpha-ethynyl-2-deoxynucleosides and 3-oxobicyclo[3.1.0]hexane dideoxynucleosides as novel templates for antiretroviral therapy.